Fluid vehicle dampers, such as hydraulic shock absorbers and struts, provide a smooth ride by absorbing forces that are generated by an uneven road surface. Two common types of vehicle fluid dampers are mono-tube and twin tube shock absorbers, each of which have a cylinder and piston with a piston rod. The piston divides the shock absorber into compression and rebound fluid chambers and regulates the flow of fluid from one chamber to another thereby achieving particular ride handling characteristics. Typically, the piston has a compression and rebound valve assembly located on opposing surfaces that regulate fluid flow during the compression and rebound strokes. By modifying the valve assemblies, the ride handling characteristics may be calibrated.
It is desirable to have different rates of piston damping depending on vehicle ride conditions. For example, during vehicle cornering maneuvers in which the piston undergoes low speed compression, it is desirable to have a stiff ride handling characteristic, which manifests itself as a steep curve on a piston force v. piston velocity graph. Conversely, when the vehicle travels over pot holes at relatively high vehicle speeds in which the piston undergoes high speed compression, it is desirable to have a soft ride handling characteristic, which manifests itself as a flat curve on a piston force v. piston velocity graph. Further, it is desirable to have independent low and high speed damping rates that may be calibrated easily. One way to best achieve independent rates of damping is to provide a piston valve assembly having digressive performance, that is, a piston valve assembly which provides generally independent and distinct damping rates during low and high speed piston compression with very steep and very flat curves, respectively. Ease of calibration may be achieved when a wide range of damping rates may be attained by varying the valve assembly components. Prior art mono-tube designs have had very limited digressive performance with interdependent low and high speed damping rates thereby compromising versatility of ride handling characteristics at low and high vehicle speeds. Therefore, what is needed is a piston valve assembly that provides increased digressive performance.